Environment variables are a set of dynamic values that may affect the way running processes will behave on a computer. In Unix and Unix-like systems, each process has its own private set of environment variables. By default, when a process is created it inherits a duplicate environment of its parent process, except for explicit changes made by the parent when it creates the child. Alternatively, from shells such as bash, an environment variable may be changed for a particular command invocation by, for example, indirectly invoking it via env or using the ENVIRONMENT_VARIABLE=VALUE<command> notation.
Regular expressions are a context-independent syntax that may represent a wide variety of character sets and character set orderings, where these character sets are interpreted according to the current locale. While many regular expressions may be interpreted differently depending on the current locale, many features, such as character class expressions, provide for contextual invariance across locales.
In computing, regular expressions may provide a concise and flexible means for identifying strings of text of interest, such as particular characters, words, or patterns of characters. Regular expressions are written in a formal language that may be interpreted by a regular expression processor, a program that either serves as a parser generator or examines text and identifies parts that match the provided specification.